Existing satellite broadcast communication systems, such as, for example, the one currently utilized by Sirius Satellite Radio, employ two forms of modulation to convey information, single carrier Quadrature Phase Shift Keying (QPSK) and multicarrier differential COFDM.
QPSK is a modulation technique that allows for the transmission of digital information across an analog channel. In QPSK, data bits are grouped into pairs with each pair represented by a particular waveform, commonly referred to as a symbol. There are four possible combinations of data bits in a pair, and a unique symbol is required for each possible combination of data bits in a pair. QPSK creates four different symbols, one for each pair, by changing the I gain and Q gain for the respective cosine and sine modulators. The symbol is then sent across an analog channel after modulating a single carrier. A receiver can demodulate the signal and look at the recovered symbol to determine which combination of data bits was sent in an original pair.
COFDM, or Coded Orthogonal Frequency-division Multiplexing (COFDM) is a frequency-division multiplexing (FDM) scheme utilized as a digital multi-carrier modulation method. A large number of closely-spaced orthogonal sub-carriers are used to carry data. The data is divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme (such as, for example, quadrature amplitude modulation (QAM) or phase shift keying (QPSK)) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth. For example, a COFDM system can distribute a single digital signal across 1,000 or more signal carriers simultaneously. Coded data is modulated and inserted into orthogonal carriers in the frequency domain. Because signals are sent at right angles to each other, the signals do not interfere with one another.
One problem that occurs in all RF transmission is multi-path effects. This refers to the scattering of a signal due to obstructions such as canyons, buildings, etc., that can cause a signal to take two or more paths to reach its final destination. COFDM is highly resistant to multi-path effects because it uses multiple carriers to transmit the same signal, making it a robust transmission method. However, the current modulation techniques used by satellite broadcast communication systems, cannot convey additional information overlaid on an COFDM signal. As overlay modulation, or multi-layer modulation is a useful and efficient method to optimize available bandwidths, the ability to overlay COFDM signals with multiple layers of modulation is highly desirable. In systems where overlay modulation is contemplated that include a COFDM transmission in addition to, for example, other transmissions, such as Time Division Multiplexing (single carrier) transmissions, it would be very useful to be able to implement the overlay technique on COFDM as well, so that the entire system can support overlay modulation.
What is thus needed in the art is an alternative implementation of COFDM that can overcome or ameliorate the problems of the prior art.